Method for transforming a substrate with edge contacts into a ball grid array

ABSTRACT

The transformation of An IC substrate with edge contacts into a ball grid array is accomplished using a flexible circuit that has terminals arranged planarly on the underside for receiving meltable solder humps and whose conductors leading outwardly from the terminals have exposed ends. The upper side of the flexible circuit is connected with the lower side of the substrate, whereupon the edge regions of the flexible circuit are bent up and around the substrate and the ends of the conductors are electrically contacted to the edge contacts of the substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a method for transforming asubstrate with edge contacts into a ball grid array, a ball grid arraymanufactured according to this method, and flexible wiring for thetransformation of a substrate with edge contacts into a ball grid array

2. Description of the Prior Art

In the manufacturing of multichip modules (MCM), i.e. systems with oneor more unencapsulated integrated circuits on a flexible circuit(conductor pattern) film, the problem arises of contacting this moduleon an element that is a continuation of the circuit. Specially formededge terminals are conventionally used that are then soldered. e.g. inthe strap soldering method, with a circuit board that continues thecircuit. As in the case of encapsulated components for surface mounting,the increasing number of terminals and the drive towards miniaturizationleads to an ever-smaller terminal grid. This in turn causes difficultiesin mounting, from the application of the soldering paste in a definedmanner to the short-circuit-free soldering with the soldering strap.

With respect to the above-named difficulties, ball grid arrays (BGAs)are increasingly being used both for individual chips and for multichipmodules, in which BGAs the terminals are distributed in planar fashionand thus in an essentially coarser grid than in the conventional formson the underside of the housing. Each of these planarly distributedterminals supports one or more humps that can be melted, known as balls,which provide the connection with the conductors (circuit) in a meltingprocess (cf. DE-Z Productronic Vol. 5, 1994, pages 54, 55).

If the wirings of the multichip modules are manufactured in circuitboard technology (known as MCM-L), the ball grid array can be realizedeasily. Through-contacts through the substrate enable a planararrangement of the terminals or of the meltable humps on the underside.Likewise, given multilayer ceramics manufactured in sinter technologywith a thick layer wiring (called MCM-C), a planar arrangement of theterminals or of the meltable humps is possible.

If, however, the multichip modules are manufactured by deposition oflayers on an unstructured substrate, e.g. ceramic (called MCM-D), thissubstrate will only outwardly have edge contacts for the connection;i.e., assembly as a ball grid array is not possible without takingfurther steps.

From PCT Application WO 89/10005, a chip packaging is known in which apreformed substrate has a receptacle at its upper side for receiving anintegrated circuit, and has four ribs on its lower side that projectbeyond the outer edge. A flexible wiring, with an opening correspondingto the receptacle, is applied on the upper side of the substrate, and isbent downwardly around the ribs such that the ends of the individualconductors form terminals in the region of the ribs. These terminals,however, are edge contacts located underneath; i.e., a realization as aball grid array is also not possible in this technique.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a methodtransforming a substrate with edge contacts into a ball grid array in asimple way.

The invention proceeds from a recognition that, based on TAB (TapeAutomated Bonding) technology, a flexible circuit (conductor pattern)film can be manufactured with planarly distributed terminals, with theconductors leading outwardly from these terminals having exposed ends,i.e. the flexible foil or film that serves as a support material iseither not present in the end region of the conductors or has beenremoved. A flexible conductor pattern fashioned in this way can then beconnected with the underside of the IC substrate, by bending the edgesof the conductor pattern up and around the sides of the substrate andthe ends of the conductors are contacted to the edge contacts on theupper side of the substrate. For manufacturing the ball grid array, itis then necessary only to apply meltable humps or balls onto theplanarly distributed terminals of the flexible conductor pattern. Thus,the flexible conductor pattern enables processing as a ball grid array,with all the advantages that this construction offers over fine-gridedge contacts, particularly in larger sizes. If needed, the entireconstruction can be covered, extrusion-coated, encapsulated, or providedwith a cap.

The support film can be provided with a slot-shaped opening at the endregions of the conductors, which enables a particularly simpleconstruction of the exposed ends of the conductors. In the realization,proven methods of TAB technology can be used.

An additional slot-shaped opening can be provided in the support film ata region of the film which will cover a lower edge of the substrate.This facilitates the bending of the flexible wiring around the loweredge of the substrate.

The conductor layout of the flexible circuit can be covered with solderresist. By means of the protective action of the solder resist, theapplication of meltable balls and also the connection of the finishedball grid array with a continuing circuit are facilitated.

An adhesive between the upper side of the flexible circuit and theunderside of the substrate enables a simple and reliable whole-surfaceconnection between the upper side of the flexible circuit and the lowerside of the substrate.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top view of a flexible circuit (conductor pattern) fortransforming a substrate with edge contacts into a ball grid array inaccordance with the invention.

FIG. 2 shows a cross-section through a portion of a substrate with edgecontacts, which is transformed into a ball grid array in accordance withthe invention using the flexible circuit shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a top view of the lower side of a flexible circuitdesignated FV as a whole, having a support film TF, a number ofterminals A arranged in a regular grid, and conductors L leadingoutwardly from these terminals A. The flexible circuit FV ismanufactured on the basis of TAB technology, and the support film TF iscorrespondingly provided with a transport perforation TP on its upperand lower edge respectively. The terminals A, which form a grid array,are arranged inside a square core region of the flexible circuit FV,this core region being indicated in FIG. 1 by a dotted line KB. In theembodiment shown in FIG. 1, the conductor layout of the flexible circuitFV is designed so that the conductors L proceeding outwardly from theterminals A are led out in groups from all four sides, and the exposedends E of the conductors respectively emerged via slot-shaped openingsSA of the support film TF. An additional slot-shaped opening ZSA isrespectively made in the support film TF between the slot-shapedopenings SA, of which there are four in all, and the core region KB. Theslot-shaped openings SA are disposed at locations in said supportingfilm TF which will come to be substantially adjacent to upper corners ofa substrate S when the support film TF is bent upwardly around the sidesof the substrate S. The additional slot-shaped openings ZSA aresubstantially parallel to the slot-shaped openings SA, and are disposedat a location which will be substantially adjacent to lower corners ofthe substrate S, when the support film TF is bent around the sides ofthe substrate S.

The support film TF is made of a heat-resistant polymer, such as e.g.polyamide. The manufacture of the conductor pattern or layout comprisingthe terminals A and the conductors L can be carried out in conventionaltechnology, such as e.g. in additive technology or in subtractivetechnology. The conductor layout consists of an electrically conductivematerial, preferably copper. The regions of the terminals A canadditionally be provided, if warranted, with a galvanically depositedtin or tin-lead layer. The slot-shaped openings S and the additionalslot-shaped openings ZSA can for example be made in the support film TFby chemical physical etching or by laser ablation. It is therebyimportant that the individual conductors L that traverse the additionalslot-shaped openings ZSA, and the ends E of the conductors L emergingvia the slot-shaped openings A, are not damaged during the removal ofmaterial.

The flexible circuit FV described above on the basis of FIG. 1 has astructure and design specifically for the purpose of transforming asubstrate with edge contacts into a ball grid array.

FIG. 2 shows a highly simplified representation of a cross-sectionthrough such a substrate, designated S as a whole, provided in all fouredge regions with edge contacts RK at a uniform spacing. On the upperside of the substrate S, which is made, for example, of ceramic, thereare one or more integrated circuits, which, like the conductors leadingoutwardly to the edge contacts RK, are not shown in FIG. 2.

For the transformation of the substrate S with the edge contacts RK intoa ball grid array BGA, first the upper side of the flexible circuit VFis connected over its whole surface with the underside of the substrateS by means of an adhesive K. This connection surface between thesubstrate S and the flexible circuit FV corresponds at leastapproximately to the core region KB shown in FIG. 1, so that the fourprojecting edge regions of the flexible circuit FV can be bent up andaround the substrate S. The upward bending is facilitated by theadditional openings ZSA and the regions of the support film TF, at thelower edges of the substrate S. In the bending of the edge regions ofthe flexible circuit FV around the upper edges of the substrate S, thefree ends E of the conductors L are pressed onto allocated edge contactsRK, so that an electrically conductive connection can now be createdbetween the ends E and the edge contacts RK. This contacting can takeplace by means of soldering with a soldering strap, by laser, infraredradiation, or the like, but the solder used must have a higher meltingpoint than the temperature occurring in the processing as a ball gridarray BGA. This contacting can also be achieved using fine weldingmethods, i.e. thermocompression, thermosonics, and ultrasound, or withadhesive methods. After the contacting, edge regions of the flexiblesupport film TF that are still protruding and are not required can beseparated if necessary.

On the underside of the flexible circuit FV adhered to the substrate S,the conductor layout can be covered with solder resist LSL except forthe planarly distributed terminals A, followed by the creation of solderpads by applying solder paste, balls, or formed solder parts and meltingthem. A more detailed explanation of the manufacturing of such solderpads can be found, for example, in German PS 195 35 622 or in U.S. Pat.No. 5,284,287.

With the application of the solder pads or balls, designated here asmeltable humps H, a processing as a ball grid array BGA is possible,with all the advantages that this constructive form offers overfine-grid edge contacts RK. If needed, the overall construction can becovered, injection-molded, encapsulated or provided with a cap. Thelast-named possibility is shown in FIG. 2 by means of a protective capdesignated SK.

In the embodiment described above, all four edge sides of the substrateare occupied with edge contacts, however, the transformation into a ballgrid array can also be carried out in an analogous fashion if only oneedge side is occupied with edge contacts, or if two or three edge sidesare occupied with edge contacts.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the inventors to embody within thepatent warranted hereon all changes and modifications as reasonably andproperly come within the scope of their contribution to the art.

We claim as our invention:
 1. A method for transforming an integratedcircuit substrate having edge contacts into a ball grid array, themethod comprising the steps of: providing said integrated circuitsubstrate with sides, edges and an upper surface, said edge contactsdisposed on said upper surface adjacent said edges; providing a flexiblecircuit having a conductor layout supported by a supporting film andhaving terminals disposed in a plane on an underside of said flexiblecircuit, said conductor layout having conductors proceeding outwardlyfrom said terminals with said conductors having exposed ends; providinga slot-shaped opening in said supporting film at a location adjacent anupper corner of said substrate when said flexible circuit is bentupwardly and around said edges of said substrate; disposing meltablesolder humps respectively at said terminals of said flexible circuit;mechanically connecting an upper side of said flexible circuit with saidunderside of said substrate with said flexible circuit projecting beyondsaid substrate at said edges of said substrate; bending said flexiblecircuit upwardly and around said edges of said substrate so that therespective exposed ends of said conductors of said flexible circuit aredisposed in registration with said edge contacts of said substrate; andproducing an electrically conductive connection between said free endsof the respective conductors of said flexible circuit and said edgecontacts of said substrate.
 2. A method as claimed in claim 1,comprising the additional step of providing a slit-shaped opening insaid supporting film of said flexible circuit at a location which willbe adjacent to a lower corner of said substrate when said flexiblecircuit is bent upwardly and around said edge regions of said substrate.3. A method as claimed in claim 1 comprising the additional step ofcovering said conductor layout of said flexible circuit with a solderresist, except at said terminals.
 4. A method as claimed in claim 1wherein the step of mechanically connecting said upper side of saidflexible circuit with said underside of said substrate comprisesattaching said upper side of said flexible circuit to said underside ofsaid substrate with an adhesive.
 5. A method for transforming anintegrated circuit substrate having edge contacts into a ball gridarray, the method comprising the steps of: providing said integratedcircuit substrate with sides, edges and an upper surface, said edgecontacts disposed on said upper surface adjacent said edges; providing aflexible circuit having a conductor layout supported by a supportingfilm and having terminals disposed in a plane on an underside of saidflexible circuit, said conductor layout having conductors proceedingoutwardly from said terminals with said conductors having exposed ends;providing a first slot-shaped opening in said supporting film at aregion adjacent an upper corner of said substrate when said flexiblecircuit is bent upwardly and around said substrate; providing a secondslot-shaped opening in said supporting film at a region adjacent a lowercorner of said substrate when said flexible circuit is bent upwardly andaround of said substrate; disposing meltable solder humps respectivelyat said terminals of said flexible circuit; mechanically connecting anupper side of said flexible circuit with said underside of saidsubstrate with said flexible circuit projecting beyond said substrate atsaid edges of said substrate; bending said flexible circuit upwardly andaround said edges of said substrate so that the respective exposed endsof said conductors of said flexible circuit are disposed in registrationwith said edge contacts of said substrate; and producing an electricallyconductive connection between said free ends of the respectiveconductors of said flexible circuit and said edge contacts of saidsubstrate.